Oldest mephitine cranium and its implications for the ... · 224 Oldest mephitine cranium: WOLSAN...

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Oldest mephitine cranium and its implications for the origin of skunks MIECZYSEAW WOLSAN Wolsan, M. 1999. Oldest mephitine cranium and its implications for the origin of skunks. - Acta Palaeontologica Polonica 44,2,223-230. Recent molecular studies have recognized the skunks (Mephitinae)to be the sister taxon -- of a clade comprising the Procyonidae and Mustelidae. These findings are inconsistent with the traditional placement of the skunks among mustelids, which is based on mor- phological evidence from extant taxa. This paper reports on a well-preserved cranium of a young individual, recovered from the middle Miocene deposits (MN 7+8, about 11-12 Ma) of Steinheim am Albuch, Germany. The fossil is the holotype of Palaeo- mephitis steinheimensis Jager, 1839, which is here recognized as a senior subjective syn- onym of Trochotherium cyamoides Fraas, 1870 (consequently, Palaeomephitis Jager, 1839 is a senior subjective synonym of Trochotherium Fraas, 1870). The specimen is identified as the oldest and most primitive mephitine cranium known to date, approach- ing the primitive morphology for the Mephitinae. It exhibits a combination of mephitine (accessory middle-ear chamber, lateral swelling of the squamosal) and mustelid (mus- telid suprameatalfossa) synapomorphies,corroboratingthe view that skunks are derived from a mustelid ancestor. Its auditory bulla shows a slightly inflated and relatively large caudal entotympanic, which indicates that the uninflated and relatively small caudal entotympanics of adult mephitines, as well as their hypertrophied ectotympanics, are not primitive (as hitherto assumed) but derived, providing a synapomorphy that supports a sister-group relationship between the Mephitinae and Lutrinae. Key words : Palaeomephitis, Trochotherium, Mephitinae, Mustelidae, Carnivora, phy- logeny, taxonomy, morphology. Mieczystaw Wolsan [[email protected]], Instytut Paleobiologii PAN, ul. Twarda 51/55, PL-00-818 Warszawa, Poland. Introduction The skunks (Mephitinae, including stink badgers, Mydaus) have traditionally been in- cluded in the carnivoran family Mustelidae (most recently in a close relationship to the Lutrinae and Melinae), based on morphological data collected from extant taxa (Bryant et al. 1993, and references therein). Recently, however, molecular evidence

Transcript of Oldest mephitine cranium and its implications for the ... · 224 Oldest mephitine cranium: WOLSAN...

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Oldest mephitine cranium and its implications for the origin of skunks

MIECZYSEAW WOLSAN

Wolsan, M. 1999. Oldest mephitine cranium and its implications for the origin of skunks. - Acta Palaeontologica Polonica 44,2,223-230.

Recent molecular studies have recognized the skunks (Mephitinae) to be the sister taxon --

of a clade comprising the Procyonidae and Mustelidae. These findings are inconsistent with the traditional placement of the skunks among mustelids, which is based on mor- phological evidence from extant taxa. This paper reports on a well-preserved cranium of a young individual, recovered from the middle Miocene deposits (MN 7+8, about 11-12 Ma) of Steinheim am Albuch, Germany. The fossil is the holotype of Palaeo- mephitis steinheimensis Jager, 1839, which is here recognized as a senior subjective syn- onym of Trochotherium cyamoides Fraas, 1870 (consequently, Palaeomephitis Jager, 1839 is a senior subjective synonym of Trochotherium Fraas, 1870). The specimen is identified as the oldest and most primitive mephitine cranium known to date, approach- ing the primitive morphology for the Mephitinae. It exhibits a combination of mephitine (accessory middle-ear chamber, lateral swelling of the squamosal) and mustelid (mus- telid suprameatal fossa) synapomorphies, corroborating the view that skunks are derived from a mustelid ancestor. Its auditory bulla shows a slightly inflated and relatively large caudal entotympanic, which indicates that the uninflated and relatively small caudal entotympanics of adult mephitines, as well as their hypertrophied ectotympanics, are not primitive (as hitherto assumed) but derived, providing a synapomorphy that supports a sister-group relationship between the Mephitinae and Lutrinae.

Key words : Palaeomephitis, Trochotherium, Mephitinae, Mustelidae, Carnivora, phy- logeny, taxonomy, morphology.

Mieczystaw Wolsan [[email protected]], Instytut Paleobiologii PAN, ul. Twarda 51/55, PL-00-818 Warszawa, Poland.

Introduction

The skunks (Mephitinae, including stink badgers, Mydaus) have traditionally been in- cluded in the carnivoran family Mustelidae (most recently in a close relationship to the Lutrinae and Melinae), based on morphological data collected from extant taxa (Bryant et al. 1993, and references therein). Recently, however, molecular evidence

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has been presented (Ledje & h a s o n 1996a, 1996b; Dragoo & Honeycutt 1997), ar- guing for a sister-group relationship between the Mephitinae and a clade containing the Procyonidae and Mustelidae. Because the two databases derived from extant taxa have provided inconsistent results, phylogenetic information from early members of the Mephitinae is essential for solving the problem of mephitine origins, especially as the fossil record is the only source of direct evidence of past organismal history.

Here I report a well-preserved cranium recovered from the lacustrine deposits of Steinheim am Albuch, southern Germby. This locality has yielded a classic middle Miocene floral and faunal assemblage (Heizmann 1976) referred to MN zone 7+8 (Bruijn et al. 1992) which corresponds to the interval of 11.2-12.5 Ma (Steininger et al. 1996). The other hitherto recorded Tertiary crania of skunks are about 3-9 million years younger and come from upper Miocene and Pliocene strata of Eurasia (Gaudry 1862; Alexejew 1916; Pilgrim 1933; Zdansky 1937; Krokos 1939; He & Huang 1991). The Steinheim specimen represents the most primitive mephitine cranium known to date, plausibly approaching the primitive morphology for this group. It exhibits fea- tures neglected in the previous descriptions of the cranium (by Jager 1839, Fraas 1870, and Helbing 1936), proving its mephitine status and shedding new light on the rela- tionships of the skunks. These features are presented in this paper.

Institutional abbreviations: AMNH, Department of Vertebrate Paleontology, Amer- ican Museum of Natural History, New York, USA; BMNH, The Natural History Museum, London, UK; BSP, Bayerische Staatssammlung fur Palaontologie und histo- rische Geologie, Munich, Germany; FSL, Centre des Sciences de la Terre, Universitk Lyon I, Lyon, France; FSP, Laboratoire de Gkobiologie, Biochronologie et Palkonto- logie humaine, Universitk de Poitiers, Poitiers, France; ISEZ, Instytut Systematyki i Ewolucji Zwierzqt PAN, Cracow, Poland; NMB, Naturhistorisches Museum, Basle, Switzerland; PVPH, Laboratoire de Palkontologie des Vertkbrks et Palkontologie humaine, Universitk Paris VI, Paris, France; SMNS, Staatliches Museum fur Natur- kunde, Stuttgart, Germany; YPM-PU, Yale Peabody Museum (Princeton University Collection), New Haven, USA; ZM, Zoologisk Museum, Karbenhavns Universitet, Copenhagen, Denmark.

Taxonomic history and status

The reported cranium (Fig. 1A-C) was collected over 160 years ago and first described by Jager (1839), who found it similar to those of extant Conepatus (a part of his Mephitis) and Mephitis, and accordingly named the fossil Palaeomephitis Stein- heimensis. Fraas (1870) and Helbing (1936) redescribed the specimen; the former be- lieved it represented a viverrid (his Viverra Steinheimensis), while the latter referred it to the alleged mephitine Trocharion albanense. Qiu & Schmidt-Kittler (1982) identi- fied Trocharion as a leptarctine mustelid, and they (Schmidt-Kittler 198 1 ; Qiu & Schmidt-Kittler 1982) consequently excluded the cranium from placement in this ge- nus, assuming its affiliation with Trochotherium cyamoides, originally described by Fraas (1 870) from the same deposits and also recorded from a few other European lo- calities of middle Miocene age (Wegner 1913; Viret 1935; Wolsan & Engesser 1997).

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ACTA PALAEONTOLOGICA POLONICA (44) (2) 225

Fig. 1. Holotype cranium (SMNS 4743a) of Palaeomephitis steinheimensis in dorsal (A), posterior (B), and ventral (C) views, its left ear region in ventral view (D), and its right ear region (E) and a radiograph of this region (F) in ventral view. Abbreviations: ac, accessory chamber of the middle ear; am, roof of the external auditory meatus; ce, caudal entotympanic; ec, ectotympanic; er, epitympanic recess; Is, lateral swelling of the squamosal; m, mastoid; mp, mastoid process; re, rostral entotympanic; s, squamosal; sf, suprameatal fossa. White triangles indicate the suture between the rostral and caudal entotympanics (D, left) and an ap- proximate border (the suture is fused and partly obliterated) between the caudal entotympanic and ectotympanic @, right). Scale bar: A-C, 20 rnm; D-F, 9 mm.

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The remains unquestionably referred to Trochotherium cyamoides consist of upper and lower teeth and partial maxillaries and dentaries (Fraas 1870,1885; Helbing 1936; Wegner 1913; Viret 1935; Heizmann 1973). They show a number of derived charac- ters shared with the living skunks, which supports the view that Trochotherium cyamoides is a mephitine. These characters include: the lack of the upper second molar and upper and lower first premolars; the protocone wing of the upper fourth premolar shelf-like and positioned at the level of the paracone; the upper first molar larger than the upper fourth premolar; no metaconule on the upper first molar and no posterior ac- cessory cusp on the lower fourth premolar; the lower first molar with more than two roots; the lower second molar single-rooted.

A derived cranial character shared by all mephitines and exclusive to this group is the extension of the epitympanic recess into the mastoid and the posterior portion of the squamosal, forming an accessory chamber in the middle ear (Schmidt-Kittler 1981; Bryant et al. 1993). This character is associated in all skunks with the derived lateral swelling of the squamosal dorsal to the mastoid process (Bryant et al. 1993). The two unique synapomorphies of the Mephitinae are present in the cranium of Palaeomephitis steinheimensis (Fig. lB, F), proving its mephitine status. The acces- sory middle-ear chamber is proportionally smaller (more primitive) in this cranium than it is in the living skunks.

Their mephitine status, presence at the same locality, and the fact that the remains attributed to Trochotherium cyamoides correspond in size to the cranium of Palaeo- mephitis steinheimensis support the view that the two names represent the same spe- cies. This view has already been expressed by Wolsan (1993a) and Wolsan & Engesser (1997), but no evidence was provided. Based on the evidence presented here, I for- mally synonymize Trochotherium cyamoides Fraas, 1870 with Palaeomephitis stein- heimensis Jager, 1839. Because these are names of the type species of their respective genera, I also synonymize the generic names Trochotherium Fraas, 1870 and Palaeo- mephitis Jager, 1839. Palaeomephitis steinheimensis and Palaeomephitis are the valid names of the species and genus, respectively, in conformity with the Principle of Prior- ity (International Commission on Zoological Nomenclature 1985: Article 23).

Implications for the relationships of skunks

All extant mustelids except mephitines possess a suprameatal fossa that is partially or completely closed ventrally and anteriorly by the ossified tube of the external auditory meatus (Schmidt-Kittler 1981). The fossil record shows that this middle-ear fossa evolved from the entirely open deep suprameatal fossa of early mustelids in which the meatal tube was poorly developed (Schmidt-Kittler 198 1 ; Wolsan 1993a). An open, deep suprameatal fossa is also characteristic of procyonids (Schmidt-Kittler 1981), but (contrary to Schmidt-Kittler 1981) it displays an essentially different configuration and its origin is distinct from that in mustelids. As evidenced by crania of Oligocene and early Miocene musteloids, the procyonid suprameatal fossa originated in consequence of a deep dorsal expansion of the primitively shallow fossa seen in stem musteloids (Bavarictis, Brachypsalis, Mustelavus, Mustelictis, Oligobunis, Promartes) and ailurids (Ailurus, Amphictis, Simocyon), and thereby preserved about equally extended lateral

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ACTA PALAEONTOLOGICA POLONICA (44) (2) 227

and medial walls (Wolsan 1992,1993a, 1993b, 1994, 1996,1997a, 1997b; Wolsan & Lange-Badre 1996). In addition, the suprameatal fossa and epitympanic recess are about equally extended dorsally (apomorphy), and the epitympanic recess is laterally exca- vated into the squamosal in front of the suprameatal fossa (apomorphy) because there is no place for it over the fossa (Wolsan 1998). In contrast, the mustelid suprameatal fossa developed from the primitive shallow fossa through ventral elongation of its lateral wall, so that the lateral wall apomorphically extends further ventrally than the medial wall, which may not be differentiated at all (Wolsan 1992,1993a, 1993b, 1994,1996; Wolsan & Lange-Badre 1996). The mustelid suprameatal fossa extends less dorsally than the epitympanic recess (plesiomorphy), and the epitympanic recess is laterally excavated into the squamosal over the suprameatal fossa (plesiomorphy). The early mustelid ar- rangement is unknown outside of the Mustelidae. This arrangement is also seen in Palaeomephitis steinheimensis (Fig. lE), which indicates that the mustelid suprameatal fossa was primitively present in the Mephitinae, and therefore argues for the mustelid status of the skunks. In the reported cranium the suprameatal fossa is posteriorly deeply excavated into and partly floored by the medial part of the mastoid process, its medial wall is not differentiated, and the anterior and lateral walls are about perpendicular to the meatal roof (Fig. 1E). This form of the suprameatal fossa conforms to that exhibited by the oldest and most primitive known, completely developed mustelid suprameatal fossae (seen in late Oligocene members of Plesictis: AMNH 1 1001, NMB Bst3853, NMB Cod2181), with the exception that the lateral wall of the fossa in Palaeomephitis steinheimensis is less extended ventrally and the anterior wall is better developed. The loss of the suprameatal fossa within the Mephitinae was apparently caused by the devel- opment of a long ectotympanic tube of the external auditory meatus, which is a charac- teristic feature of late Miocene to Recent skunks. In Palaeomephitis steinheimensis this tube is still short and incompletely developed, so that the meatus is roofed by the squamosal (Fig. ID, E).

Among extant mustelids, the mephitines and lutrines are unique in having auditory bullae with an uninflated caudal entotympanic that is small relative to the ecto- tympanic (which is hypertrophied) and rostra1 entotympanic (Hunt 1974). This config- uration has generally been considered primitive, following Hunt (1974), who relied on general evidence from carnivoran ontogeny. However, in the cranium of Palaeo- mephitis steinheimensis the caudal entotympanic is slightly inflated and much larger, and the ectotympanic is less well developed (Fig. ID). The size relations between the two bones approach those seen among stem musteloids (e.g., the late Oligocene Bavarictis: BSP 1952115) and in the earliest known ailurids (the late Oligocene to mid- dle Miocene Amphictis: FSP PFRA28, ISEZ MF2130), the earliest known procyonids (the late Oligocene Pseudobassaris: BMNH M9647, PVPH PVQ70-2, YPM-PU 11455, ZM 144), and the earliest known mustelids (the late Oligocene to early Mio-

, cene Plesictis: AMNH 11001, FSL 97448, NMB Bst3853, NMB Cod21 81). Although the ossification of the reported cranium had not been completed before the end of the animal's life (as evidenced by numerous unfused sutures between bones; Fig. 1A-E), and therefore some ontogenetic changes in inflation and size proportions between the bullar bones could have occurred if the animal had lived longer, it is evident that the uninflated and small caudal entotympanics of late Miocene to Recent skunks, as well as their hypertrophied ectotympanics, are not primitive but derived. As the adult mor-

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phology of the auditory bulla in Palaeomephitis steinheimensis is uncertain, and in all other known mephitines the apomorphic condition is present, I consider this condition apomorphic for the whole subfamily and therefore regard it as supporting a sis- ter-group relationship between the Mephitinae and Lutrinae.

Conclusions

Palaeomephitis steinheimensis Jager, 1839 is a senior subjective synonym of Tro- chotherium cyamoides Fraas, 1870 and the valid name for the species. Palaeomephitis Jager, 1839 is a senior subjective synonym of Trochotherium Fraas, 1870 and the valid name for the genus. Palaeomephitis steinheimensis is a mephitine. The mephitines are mustelids. The Mephitinae and Lutrinae are sister taxa.

Acknowledgements

I thank L. de Bonis (FSP), B. Engesser (NMB), V. Fahlbusch and K. Heissig (BSP), J.A. Gauthier and M.A. Turner (YPM-PU), T. Hatting (ZM), E.P.J. Heizmann (SMNS), J.J. Hooker (BMNH), B. Lange-Badre (PVPH), A. Nadachowski (ISEZ), A. Prieur (FSL), and R.H. Tedford (AMNH) for access to the specimens and assistance. Special thanks are extended to L. Werdelin for suggestions to improve the manuscript. R. Harling took the photographs, and R. Wild provided the radiograph. This work was supported by grant 6 P04C 019 08 from the State Committee for Scientific Research (KBN).

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Najstarsza m6zgoczaszka przedstawiciela skunks6w i jej znaczenie dla wyjahienia pochodzenia tej gmpy

MIECZYSEAW WOLSAN

S treszczenie

Pochodzenie skunks6w (Mephitinae) jest obecnie przedmiotem kontrowersji. Podczas gdy badania morfologiczne wsp6lczesnych takson6w potwierdzajq tradycyjne umiejscowienie skunks6w w obrqbie ssak6w drapieznych z rodziny lasicowatych (Mustelidae), w bliskim pokrewieiistwie do wydr (Lutrinae) i borsuk6w (Melinae), to wyniki badaii molekularnych sugerujq, ze skunksy sq grupq siostrzanq kladu obejmujqcego lasicowate i szopowate (Pro- cyonidae). W tej sytuacji sprzecznoSci pomiqdzy wynikami uzyskanymi na podstawie obu baz danych opartych na zyjqcych taksonach, dane paleontologiczne z wczesnych etap6w ewolucji skunks6w mogq mieC znaczenie decydujqce dla ustalenia pozycji filogenetycznej tej podrodziny .

Najstarszq i najbardziej prymitywnq znanq m6zgoczaszkq skunksa jest holotyp gatunku Palaeomephitis steinheimensis Jager, 1839 (Fig. 1A-C). M6zgoczaszkq tq wydobyto ze Srod- kowomioceiiskich (MN 7+8, ok. 11-12 mln lat temu) osad6w Steinheim am Albuch w polud- niowych Niemczech. Reprezentuje ona Idodego osobnika, o czym Swiadczq liczne niezroS- niqte szwy miqdzy koSCmi oraz uksztaltowanie powierzchni koSci (Fig. 1A-E).

Za mlodszy subiektywny synonim Palaeomephitis steinheimensis uznano tutaj Trocho- therium cyamoides Fraas, 1870, opisane z tych samych osad6w i znane r6wniez z kilku innych Srodkowomiocefiskich stanowisk w Europie. Poniewaz obie nazwy dotyczq gatunk6w typo- wych swoich rodzajbw, Trochotherium Fraas, 1870 jest dodszym subiektywnym synoni- mem nazwy Palaeomephitis Jager, 1839.

M6zgoczaszka Palaeomephitis steinheimensis posiada dodatkowq komorq w uchu Srod- kowym (Fig. IF: ac) i bocznq wypuMoSC koSci luskowej ponad wyrostkiem sutkowym (Fig. 1B: 1s). Cechy te sq unikalnymi synapomorfiami skunks6w i Swiadczq o przynaleznoSci Palaeomephitis steinheimensis do tej podrodziny. Chociaz u wsp6lczesnych skunks6w brak jest w uchu Srodkowym d o h nadprzewodowego (fossa suprameatale), w m6zgoczaszce Palaeomephitis steinheimensis zaglqbienie to wystqpuje (Fig. 1E: sf) i jest wyksztalcone w spos6b charakterystyczny dla najstarszych znanych lasicowatych i synapomorficzny dla tej rodziny. Wskazuje to na plezjomorficznq obecnoSC tej cechy w obrqbie podrodziny skunksdw i przemawia za ich przynaleznoSciq do lasicowatych.

SpoSr6d wsp6lczesnych lasicowatych, jedynie u skunks6w i wydr koSC wewnqtrzbqben- kowa tylna (entotympanicum caudale) nie jest wydqta i jest mala w stosunku do koSci zew- nqtrzbqbenkowej (ectotympanicum), kt6ra jest silnie rozwiniqta, i koici wewnqtrzbqben- kowej przedniej (entotympanicum rostrale). Talc uksztaltowane puszki bebenkowe skunks6w i wydr traktowano dotqd jako cechq plezjomorficznq. W m6zgoczaszce Palaeomephitis steinheimensis koSC wewnqtrzbqbenkowa tylna jest jednak nieco wydqta i doSC duza (Fig. ID: ce), a koSC zewnqtrzbqbenkowa jest stosunkowo mala (Fig. ID: ec), zblizajqc siq wzglqdnymi rozmiarami do stosunk6w panujqcych u najstarszych znanych lasicowatych, szopowatych i mdych pand (Ailuridae), a takie w wymarlych rodzajach z pnia tych rodzin. Przemawia to za apomorficznoSciq uksztaltowania puszki bqbenkowej skunks6w, dostarczajqc w ten spo- s6b synapomorfii na poparcie siostrzanego pokrewieiistwa pomiqdzy podrodzinami skunk- s6w i wydr.